This application relates to telecommunications and more particularly to radio communication systems in which distances between transmitters and receivers can vary.
In a communication system in which the ranges between transmitters and receivers can vary, the effective sensitivity of a receiver can be reduced by high-power transmissions from nearby transmitters. The receiver""s effective sensitivity is the minimum received signal strength (RSS) that permits the received signal to be processed successfully. For example in a cellular radio telephone communication system, the effective sensitivity of a base station to signals from a mobile station can be reduced (i.e., the minimum RSS can be increased) in the presence of strong signals from mobile stations on other frequencies or channels (adjacent-channel interference) and/or on the same frequency or channel (co-channel interference).
Such sensitivity reduction can result from many causes in the receiver, including intermodulation products generated during radio signal downconversion and demodulation that increase the probability of errors at the base station and the potential for dropped connections. The effects of signal distortions like intermodulation products are sometimes handled by suitable digital processing, as described for example in U.S. Pat. No. 5,768,307 to Schramm et al.
A reduction of the sensitivity of a receiver such as a base station will have its strongest effect on distant mobile stations since the received signal strengths of those mobile stations are likely to be close to the base station receiver""s sensitivity. Such problems caused by signals being received at widely varying power levels are particularly acute in multi-layer, or hierarchical, cellular systems, in which large macrocells overlie smaller microcells that may themselves overlie even smaller picocells. Such systems are known in the art, for example U.S. Pat. No. 5,572,221 to Marlevi et al., which is incorporated here by reference. Such multi-layer systems make it more likely that a transmitter that causes a disturbance is not under the control of a cell in which the disturbance occurs. These problems also occur in areas covered by overlapping communication systems controlled by independent operators.
At the same time that a receiver must have good sensitivity, enabling it to handle low-power signals, the receiver should be able to handle high-power signals without distortion or overloading. The result of the simultaneous requirements for good sensitivity and high-power signal handling is that the receiver should have a wide dynamic range. This is usually achieved in current communication systems by arranging the amplifiers and other components of the receiver to provide a total input dynamic range on the order of 120 dB, which is usually obtained in two steps: a fixed component of about 100 dB and a variable-gain component of about 20 dB. The variable-gain range is usually provided by an amplifier whose gain can be controllably varied through the 20 dB range, and such an amplifier is usually disposed in an automatic gain control (AGC) loop.
The general arrangement of such a receiver is illustrated by FIG. 1, which shows an antenna 1 that provides a signal to a downconverter 2. As its name suggests, the downconverter shifts the signal received by the antenna to lower frequencies that are more conveniently handled. The downconverted signal is appropriately shaped by bandpass filter 3 and amplifier 4, which usually has an adjustable gain. The gain of the amplifier 4 is automatically controlled by an AGC detector and associated circuitry 5 based on feedback of a portion of the amplified downconverted signal, which in modern receivers is usually provided to an analog to digital converter 6 and further processing components. A main purpose of the AGC is to maintain the signal at a level that can be usefully handled by the analog to digital converter. This form of receiver having AGC is common in the art, as illustrated for example by FIG. 5 of U.S. Pat. No. 5,265,119 that also shows use of the feedback signal for controlling the power level of a transmitted signal.
To help limit the dynamic range requirement and deal with other problems not relevant to this application, current cellular systems, which are illustrated by FIG. 2, control the power levels of the various transmitters in the systems. In FIG. 2, a plurality of cells C1-C10 include respective base stations B1-B10 that transmit signals to mobile stations M1-M9. The base stations B1-B10 are connected to a mobile services switching center MSC that is the interface between the cellular system and another communication system (not shown) like a public switched telephone network (PSTN).
Power control as described in U.S. Pat. No. 4,485,486 to Webb et al. involves the base station""s measuring the strength of signals received from the mobile station, comparing the received signal strength with upper and lower thresholds that indicate a desired range for proper reception, and issuing a power adjustment order to the mobile station based on the comparison.
In U.S. Pat. No. 4,811,421 to Havel et al., the mobile station determines its transmission power level such that the strength of the signal received by the base station will be constant. The determination is based on strength measurements of the signal received by the mobile station that are used to estimate the path loss to the base station.
In U.S. Pat. No. 4,580,262 to Naylor et al., the output power of a transmitter is controlled by the receiver such that the output power is just enough for the communication link to have sufficient quality. The quality target is fixed, however, and thus the system suffers from a xe2x80x9cparty effectxe2x80x9d, by which all transmitters tend to increase their power levels. U.S. Pat. No. 5,574,982 to Almgren et al. describes a communication system having quality-based transmit power control that is not subject to the xe2x80x9cparty effectxe2x80x9d but that still maximizes the carrier to interference (C/I) ratio. The Almgren patent is expressly incorporated here by reference. U.S. patent application Ser. No. 08/870,867 filed on Jun. 6, 1997, by Butovitsch et al. for xe2x80x9ctransmit Power Control in a Radio Communication Systemxe2x80x9d is also expressly incorporated here by reference.
In quality-based transmit power control, the receiver determines the interference level based on a received signal quality measurement such as bit error rate (BER), frame error rate, etc. Generally this is done by comparing received versions of a predetermined signal to the expected predetermined signal and then counting deviations. For example, in a time division multiple access (TDMA) radio telephone system such as that specified by the EIA/TIA IS-54-B standard, the receiver compares a received synchronization word (SYNC) and/or a digital verification color code (DVCC) to the known SYNC or DVCC and counts the erroneous received values as a function of time. The measured received signal quality is used to generate commands that the receiver sends to the transmitter for adjusting the transmitter""s power level and thereby adjusting the received signal quality. Such adjustments are possible, however, only for transmitters that are coordinated by the cell, e.g., mobile stations that are controlled by the cell served by the (base station) receiver (e.g., mobile stations registered in the cell).
Besides these power control considerations, base stations are also subject to physical-size constraints, particularly base stations that are used in small cells, such as indoor systems. In order to build a base station that is physically small, the power consumption of the base station generally has to be small. Since the power consumed by a base station is proportional to, among other things, the dynamic range of the base station receiver, decreasing the dynamic range of the base station receiver is a way to decrease the power consumption of the base station.
The dynamic range of an amplifier determines how much the input signals to the amplifier may vary in amplitude. One way to decrease the dynamic range of a receiver is to decrease the fixed dynamic range of the variable-gain amplifier. For instance, an amplifier handling signals that can be spread in amplitude over a range of 40 dB typically consumes less power than an amplifier handling signals that can range over 100 dB, which is the range usually employed by receivers in cellular telephone systems as noted above.
As noted above, it is generally desirable in a communication system in which signals have different power levels to use a receiver having a wide dynamic range but the problem is that a high dynamic range implies a high power consumption, which in turn implies large physical size for cooling. Although reducing the dynamic range is one approach to this problem, a receiver having a reduced dynamic range suffers other problems. If the dynamic range is set to handle low-strength signals, then the receiver cannot handle high-strength signals. If the dynamic range is set to handle high-strength signals, then the receiver cannot handle low-strength signals.
One solution to this dilemma that is used in prior communication systems is to use AGC in a receiver with a reduced fixed dynamic range. The AGC is provided by an amplifier having variable gain. Nevertheless, this solution does not permit weak and strong signals to be handled simultaneously, which is important because some received signals may emanate from transmitters that are coordinated by the communication system (cell) and other received signals may emanate from transmitters that are not coordinated by the system, as noted above.
What is needed is a receiver that has a reduced dynamic range and that is still able to cope with all signals within its band, even signals from transmitters that are not coordinated by the system of which the receiver is a part. Such a receiver can be used in a communication network employing quality-based transmit power control to provide a system capable of achieving an optimal C/I ratio for all connections.
The present invention provides a receiver that combines being operable in a full input dynamic range with a smaller physical size and lower electric power consumption than prior receivers. Such a receiver is ideally suited for indoor cellular radio communication systems, among many other uses. In general, these advantages are provided by reducing the fixed component of the dynamic range and increasing the variable-gain component of the dynamic range.
In an exemplary embodiment of the invention, there is provided an apparatus in a receiver in a communication system, in which a transmitter in the communication system has a transmit power level that is controlled based on at least one signal received by the receiver. The apparatus includes an amplifier having a fixed dynamic range that is substantially less than 100 dB, an analog to digital (A/D) converter, and a gain control unit. The fixed dynamic range of the amplifier is selectively positioned in a range of at least about 80 dB in response to a feedback control signal from the gain control unit. Further, the transmit power level of one or more desired signals received by the apparatus is controlled based on the received signals.
In another aspect of the invention, a method of processing signals in a receiver in a base station in a cellular radio telephone system having a plurality of mobile stations is provided. The method includes the steps of receiving the signals, which include at least one desired signal may include at least one interfering signal, amplifying the signals with an amplifier that has a fixed dynamic range substantially less than 100 dB, and controlling a transmit power level of the at least one desired signal received at the base station based on power levels of the received signals. During the step of amplifying, the position of the dynamic range of the amplifier is selectively controlled.
In yet another aspect, in a cellular radio telephone system having a base station and a plurality of mobile stations, a method of processing received signals in the base station comprises the steps of: receiving a signal from a mobile station operating in a transmit power control loop that includes a receiver in the base station; receiving a disturbing signal from another radio transmitter having a transmit power level that is not coordinated with a transmit power level of the mobile station; moving a dynamic range of the receiver in a manner that accounts for the disturbing signal; and adjusting a transmit power level of the mobile station to maintain an acceptable received signal quality.
In yet a further aspect of the invention, there is provided a communication system that includes a receiver having a fixed dynamic range that is substantially less than 100 dB; at least one transmitter that sends a desired communication signal to the receiver; a gain control unit in the receiver that selectively moves the dynamic range when at least one of a level of the at least one desired communication signal at the receiver and a level of at least one other communication signal at the receiver is out of the fixed dynamic range of the receiver; and a device for controlling a transmit power level of the at least one desired communication signal based on power levels of the at least one desired communication signal and the at least one other communication signal at the receiver.